This invention is generally directed to an elctrostatographic imaging device containing a polyvinylsilicate overcoating; and more specifically, the present invention is directed to polyvinylsilicate coatings and photoconductive devices, especially layered photoconductive devices containing such coatings. The polyvinyl silicate coatings, and photoconductive devices containing these coatings are resistant to ozone, and to other reactive chemical substances produced by corona charging devices; and further such coatings are substantially insoluble in most solvents, thus allowing them to be formed as a discrete layer, which layer does not affect the intrinsic properties of the photoreceptor device being protected. Further, the polyvinyl silicate coatings of the present invention can be fabricated as thin films in view of their extremely high wear resistance. Also such coatings function as a release material, allowing the excellent release and transfer of toner images from photoconductive devices. Additionally, in certain liquid ink xerographic development processes the polyvinylsilicate coatings remain essentially non-reactive to the ink/solvent formulation utilized for development.
It is known that the application of protective coatings to certain photoconductive materials, particularly inorganic photoconductive materials, is designed primarily for the purpose of extending the useful life of such devices. Generally in order for these coatings to provide the desired protection they must be applied in a substantially uniform thickness. Additionally, the coating material should be selected so as to not adversely effect the photoelectric properties of the photoreceptor, for example, the coating should not appreciably inject charges in the dark. The protective coastings should also not conduct laterally on the overcoat surface. Further, in some applications the coating must be transparent, and possess a dark resistivity at least equal to the dark resistivity of the photoconductive material. For example, photoconductive materials such as selenium have a resistivity in the dark of 10.sup.10 -10.sup.12 ohm-cm, thus the dark resistivity of the protective coating should be within this range when such a coating is used as a protectant for selenium.
One of the most widely used photoconductive materials is purified vitrious selenium, however, it suffers from two serious defects, namely, its spectral response is somewhat toward the blue or near ultraviolet, and the preparation of uniform films of vitreous selenium has required highly complex processes wherein critical parameters are involved. Accordingly, from a commerical economic aspect, it is important that xerographic selenium devices be utilized for numerous imaging cycles, as is presently practiced in the art.
Deterioration by the mechanical abrasion attendant to the developing and the cleaning processes, wherein in one cleaning process a rapidly rotating brush contacts the photoconductive surface for the purpose of removing therefrom any residual developer particles adhering thereto subsequent to the transfer step, has been observed in selenium. In addition to mechanical abrasion, the selenium photoreceptor may be subjected to intense heat, which over a period of time adversely affects its photoconductivity. In view of this, and for other reasons, various protective coatings, or overcoatings have been applied to selenium devices. Thus, there is described in U.S. Pat. No. 3,397,982 an electrostatographic device comprising a photoconductive layer including an inorganic glass material, the photoconductive layer containing an overcoating comprised of various oxides, such as germanium oxides, the oxides of vanadium, and silicon dioxide.
Additionally, in U.S. Pat. No. 2,886,434 there is disclosed processes for the protecting of selenium photoconductive substances with a thin transparent film of a material having electrical characteristics equal to selenium. Examples of materials disclosed as a protective layer for selenium include zinc sulfide, silica, various silicates, alkaline earth fluorides, and the like.
Furthermore, there is disclosed in U.S. Pat. No. 2,879,360 a photoconductive cell comprising a support substrate, a layer of photoconductive material, and as a protectant, a thin film of silicon dioxide superimposed upon the photoconductive layer.
Recently, there has been developed for use in xerographic imaging systems overcoated organic imaging members, including layered organic and layered inorganic photoresponsive devices. In one such photoresponsive device, there is employed a conductive substrate, overcoated with a hole injecting layer, which in turn is overcoated with a hole transport layer, followed by an overcoating of a carrier generating layer, and an insulating organic resin overcoating as a top coating. These devices have been found to be very useful in various imaging systems, and have the advantage that high quality images are obtained with the overcoating acting primarily as a protectant. The details of this type of overcoated photoreceptor are fully disclosed in U.S. Pat. No. 4,251,612 on a dielectric overcoated photoresponsive imaging member and imaging method.
Another similar overcoated photoresponsive device is comprised of a conductive substrate layer, a generating layer, and a transport layer. In such devices the generating layer can be overcoated on the transport layer, or the transport layer may be overcoated on the generating layer. Examples of such devices are described in U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference.
Several of the above-described overcoated organic photoresponsive devices are not protected after extended usage, and in some instances the imaging properties of these devices are adversely effected after a few imaging cycles. More specifically, with such devices the properties of the top overcoating material, or the properties of the other layers are adversely effected by ozone, and other contaminants contained in the environment; by the developing compositions which contact the photoresponsive device for the purpose of rendering the image visible, and mechanical abrasion during cycling. Accordingly, images of low quality, or no images whatsoever are produced depending upon the extensiveness of the damage caused to the layers of the photoconductive device. Furthermore, in some instances, the toner materials employed do not sufficiently release from the photoresponsive surface, leaving unwanted toner particles thereon, causing such particles to be subsequently embedded into, or transferred from the imaging surface in later imaging steps, thereby resulting in undesirable images of low quality, and/or high background. Also, in some instances, the dried toner particles adhere to the imaging member and print out as background areas due to the adhesive attraction of the toner particles to the photoreceptor surface. This can be particularly troublesome when known silicone resins or elastomeric polymers are employed as overcoat materials for their melted toner release characteristics, since any low molecular weight components contained in these polymers can migrate to the surface of the silicone polymer layer, and act as an adhesive toward dry toner particles brought in contact therewith during image development. There thus results undesirable high background areas in the final image since toner particles together with the toner images are effectively transferred to the receiving sheet.
Accordingly, there continues to be a need for protective overcoatings for use in photoconductive devices, including layered devices, in which the overcoatings are chemically resistant, are substantially insoluble in most solvents, have excellent toner release properties, and can be fabricated as a thin film as a result of their extremely high wear resistance. Further, there continues to be a need for protective coatings which are of extreme hardness (superhard) or essentially equivalent to a hardness of glass, which coatings provide photoreceptor surfaces that are scratch and wear resistant. Furthermore, there continues to be a need for protective overcoatings which are (1) flexible and transparent, (2) non-reactive with, and impermeable to chemical materials produced by corona charging devices, and (3) can be easily cleaned.